As an oil/gas well or other comparably deep well is being drilled wellhead equipment is progressively assembled at the mouth, to mechanically support strings of casing and tubing run into and hung in the well, to permit mud and other fluids to be circulated into the well, to permit the controlled recovery of fluids including cuttings from the well without contaminating the water or land in the vicinity of the wellhead, to prevent sea or lake water from flowing into the well in early stages, and to control subterranean pressure. This last-mentioned purpose becomes progressively more important as the well is drilled deeper. Subterranean pressures in communication with the well bore commonly reach 5,000 p.s.i., and in deeper wells, may reach up to 30,000 p.s.i. or so.
Quite obviously it would not be feasible to construct an entire production, collection and storage system downstream of the wellhead to withstand such high pressures, especially when they may not remain steady over time, but may fluctuate somewhat erratically.
The key is to provide the wellhead equipment assembly with a pressure step-down valve, generally further upstream than all usually used but any in-the-well downhole valves, a valve which may be controlled as to its absolute degree of opening, in which in the normal case is open only slightly, in which the relative degree of opening (up to the set maximum degree) is responsive to the upstream and downstream pressures, and which may be absolutely closed. The wear conditions to which the seats and other valve surfaces between which the well fluids flow when such a valve is slightly open are extreme. Accordingly, these valves, which are called chokes have developed separately as a special sub-type of valves.
With completed, producing wells now often located at inaccessible locations it has become common to tend several wells from a common control center, using electronic pneumatic and/or hydraulic sensing, signalling and control means between each well and the common control center.
Accordingly, there is a need for chokes which can be used during drilling, completion, production and workover, which can be operated remotely, which have failsafe capabilities, and a relatively smooth curve of operating characteristics over their range of openness.